GrabDuck

How to detect when an Android app goes to the background and come back to the foreground

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UPDATE / NOTES (November 2015): People has been making two comments, first is that >= should be used instead of == because the documentation states that you shouldn't check for exact values. This is fine for most cases, but bear in mind that if you only care about doing something when the app went to the background, you will have to use == and also combine it with another solution (like Activity Lifecycle callbacks), or you may not get your desired effect. The example (and this happened to me) is that if you want to lock your app with a password screen when it goes to the background (like 1Password if you're familiar with it), you may accidentally lock your app if you run low on memory and are suddenly testing for >= TRIM_MEMORY, because Android will trigger a LOW MEMORY call and that's higher than yours. So be careful how/what you test.

Additionally, some people have asked about how to detect when you get back.

The simplest way I can think of is explained below, but since some people are unfamiliar with it, I'm adding some pseudo code right here. Assuming you have YourApplication and the MemoryBoss classes, in your class BaseActivity extends Activity (you will need to create one if you don't have one).

@Override
protected void onStart() {
    super.onStart();

    if (mApplication.wasInBackground()) {
        // HERE YOU CALL THE CODE YOU WANT TO HAPPEN ONLY ONCE WHEN YOUR APP WAS RESUMED FROM BACKGROUND
        mApplication.setWasInBackground(false);
    }
}

I recommend onStart because Dialogs can pause an activity so I bet you don't want your app to think "it went to the background" if all you did was display a full screen dialog, but your mileage may vary.

And that's all. The code in the if block will only be executed once, even if you go to another activity, the new one (that also extends BaseActivity) will report wasInBackground is false so it won't execute the code, until onMemoryTrimmed is called and the flag is set to true again.

Hope that helps.

UPDATE / NOTES (April 2015): Before you go all Copy and Paste on this code, note that I have found a couple of instances where it may not be 100% reliable and must be combined with other methods to achieve the best results. Notably, there are two known instances where the onTrimMemory call back is not guaranteed to be executed:

  1. If your phone locks the screen while your app is visible (say your device locks after nn minutes), this callback is not called (or not always) because the lockscreen is just on top, but your app is still "running" albeit covered.

  2. If your device is relatively low on memory (and under memory stress), the Operating System seems to ignore this call and go straight to more critical levels.

Now, depending how important it's for you to know when your app went to the background, you may or may not need to extend this solution together with keeping track of the activity lifecycle and whatnot.

Just keep the above in mind and have a good QA team ;)

END OF UPDATE

It may be late but there's a reliable method in Ice Cream Sandwich (API 14) and Above.

Turns out that when your app has no more visible UI, a callback is triggered. The callback, which you can implement in a custom class, is called ComponentCallbacks2 (yes, with a two). This callback is only available in API Level 14 (Ice Cream Sandwich) and above.

You basically get a call to the method:

public abstract void onTrimMemory (int level)

The Level is 20 or more specifically

public static final int TRIM_MEMORY_UI_HIDDEN

I've been testing this and it always works, because level 20 is just a "suggestion" that you might want to release some resources since your app is no longer visible.

To quote the official docs:

Level for onTrimMemory(int): the process had been showing a user interface, and is no longer doing so. Large allocations with the UI should be released at this point to allow memory to be better managed.

Of course, you should implement this to actually do what it says (purge memory that hasn't been used in certain time, clear some collections that have been sitting unused, etc. The possibilities are endless (see the official docs for other possible more critical levels).

But, the interesting thing, is that the OS is telling you: HEY, your app went to the background!

Which is exactly what you wanted to know in the first place.

How do you determine when you got back?

Well that's easy, I'm sure you have a "BaseActivity" so you can use your onResume() to flag the fact that you're back. Because the only time you will be saying you're not back is when you actually receive a call to the above onTrimMemory method.

It works. You don't get false positives. If an activity is resuming, you're back, 100% of the times. If the user goes to the back again, you get another onTrimMemory() call.

You need to suscribe your Activities (or better yet, a custom class).

The easiest way to guarantee that you always receive this is to create a simple class like this:

public class MemoryBoss implements ComponentCallbacks2 {
    @Override
    public void onConfigurationChanged(final Configuration newConfig) {
    }

    @Override
    public void onLowMemory() {
    }

    @Override
    public void onTrimMemory(final int level) {
        if (level == ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN) {
            // We're in the Background
        }
        // you might as well implement some memory cleanup here and be a nice Android dev.
    }
}

In order to use this, in your Application implementation (you have one, RIGHT?), do something like:

MemoryBoss mMemoryBoss;
@Override
public void onCreate() {
   super.onCreate();
   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
      mMemoryBoss = new MemoryBoss();
      registerComponentCallbacks(mMemoryBoss);
   } 
}

If you create an Interface you could add an else to that if and implement ComponentCallbacks (without the 2) used in anything below API 14. That callback only has the onLowMemory() method and does not get called when you go to the background, but you should use it to trim memory.

Now launch your App and press home. Your onTrimMemory(final int level) method should be called (hint: add logging).

The last step is to unregister from the callback. Probably the best place is the onTerminate() method of your App, but, that method doesn't get called on a real device:

/**
 * This method is for use in emulated process environments.  It will
 * never be called on a production Android device, where processes are
 * removed by simply killing them; no user code (including this callback)
 * is executed when doing so.
 */

So unless you really have a situation where you no longer want to be registered, you can safety ignore it, since your process is dying at OS level anyway.

If you decide to unregister at some point (if you, for example, provide a shutdown mechanism for your app to clean up and die), you can do:

unregisterComponentCallbacks(mMemoryBoss);

And that's it.